target/i386: disable VMX features if nested=0

If kvm does not support VMX feature by nested=0, the kvm_vmx_basic
can't get the right value from MSR_IA32_VMX_BASIC register, which
make qemu coredump when qemu do KVM_SET_MSRS.

The coredump info:
error: failed to set MSR 0x480 to 0x0
kvm_put_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20191206071111.12128-1-yang.zhong@intel.com>
Reported-by: Catherine Ho <catherine.hecx@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/kvm.c

@@ -2572,6 +2572,14 @@ static void kvm_msr_entry_add_vmx(X86CPU *cpu, FeatureWordArray f)
     uint64_t kvm_vmx_basic =
         kvm_arch_get_supported_msr_feature(kvm_state,
                                            MSR_IA32_VMX_BASIC);
+
+    if (!kvm_vmx_basic) {
+        /* If the kernel doesn't support VMX feature (kvm_intel.nested=0),
+         * then kvm_vmx_basic will be 0 and KVM_SET_MSR will fail.
+         */
+        return;
+    }
+
     uint64_t kvm_vmx_misc =
         kvm_arch_get_supported_msr_feature(kvm_state,
                                            MSR_IA32_VMX_MISC);